This invention relates in general to vehicular braking systems, and in particular a supply valve for use in an anti-lock braking system (ABS) including traction control (TC).
Vehicles are commonly slowed and stopped with hydraulic braking systems. These systems vary in complexity, but a base brake system typically includes a tandem master cylinder, pressure fluid conduit arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the pressure fluid conduit in both circuits to actuate wheel brakes and slow the vehicle.
Base braking systems typically use a brake booster to provide a force to the master cylinder which assists the pedal force created by the driver. The booster can be vacuum or hydraulically operated.
Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive braking pressure thus causing one or more wheels to lock, resulting in excessive slippage between the wheel and road surface. Such wheel lock-up conditions can lead to greater stopping distances and possible loss of directional control.
Advances in braking technology have led to the introduction of ABS. An anti-lock brake system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range while achieving maximum braking forces. While such systems are typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels.
Electronically controlled ABS valves, comprising apply valves and dump valves, are located between the master cylinder and the wheel brakes and perform the pressure regulation. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The apply valves allow brake pressure into the wheel brakes to increase pressure during the apply mode, and the dump valves open to release pressure from the wheel brakes during the dump mode. Wheel brake pressure is held constant during the hold mode by closing both the dump valves and the apply valves.
A further development in braking technology has led to the introduction of TC systems. Additional valves have been added to existing anti-lock braking systems to provide a braking systems which control wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this condition and automatically applies braking pressure to the wheel brakes of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, braking pressures greater than the master cylinder pressure must quickly be available when the vehicle is accelerating.
A multiplicity of control valves and hydraulic components are used in ABS/TC systems, and are typically housed in a hydraulic control unit (HCU). The HCU includes a housing having a plurality of bores in which the control valves and hydraulic components are seated. Passageways or conduits are drilled into the HCU to provide fluid communication between the various valves and components.
It is desirable to combine functions of traditional valves and hydraulic components into a single, integrated assembly. Such integrated assemblies reduce the cost of manufacturing the various components and the HCU, and also reduce the size and weight of a ABS/TC system.